Detection devices employing ultrasonic waves have been applied in, for example, medicine, nondestructive internal inspection of atomic power plants, etc. For example, because ultrasonic testing can relatively easily detect internal flaws of materials, it has been playing an important role, employed in inspection of critical parts of structural materials. In ultrasonic testing, for example, as disclosed in Patent Document 1, a piezoelectric element has been employed to transmit ultrasonic waves.
Because this piezoelectric element is relatively large, for example, having a diameter of about 20 mm, the device is also large. Accordingly, testing of narrow spaces or members having complicated shapes is difficult. In addition, because the frequency band of the ultrasonic waves is constrained by the intrinsic frequency of the piezoelectric element, it is not ideal for applications such as displaying an image of a member surface, etc.
As a way of alleviating these, for example, a laser ultrasonic method disclosed in Patent Document 2 has been proposed.
Here, laser light is radiated on a test object using an optical fiber, ultrasonic waves are generated at the surface of the test object by this laser light, and the ultrasonic waves transmitted through the test object are detected using receiving laser light. A flaw is detected by recognizing variations in these ultrasonic waves, and depth can also be determined by performing a frequency analysis of the received ultrasonic waves.
In other words, because a thin optical fiber is used in generating the ultrasonic waves, the size of the device can be reduced, and it is possible to cope with testing of narrow spaces or members having complicated shapes.
In addition, for example, it has been proposed to generate ultrasonic waves using laser light, as disclosed in Patent Document 3, and to carry out nondestructive inspection using these ultrasonic waves.
Here, laser light is radiated into a tubular object, which is closed at one end with a metal plate and whose interior is filled with gas, thereby inducing a change due to thermal expansion of the internal gas and the thermal stress of the metal plate, and the ultrasonic waves are generated by propagating this change to the outside.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-28589.    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2005-43139    Patent Document 3: the Publication of Japanese Patent No. 2984390.